Presently, ceramic materials are being used to filter gas entrained carbon particulates from the exhaust streams of diesel engines. These various filtering methods rely on the porosity of the ceramic filter as well as gas permeation through the ceramic filter walls. By the year 1994, new particulate emission standards set by the Environmental Protection Agency (EPA) will require all urban buses and heavy duty trucks to emit less than 0.1 grams per horsepower hour of particulate matter. Particulates are defined by EPA as any matter in the exhaust of an internal combustion engine, other than condensed water, which is capable of being collected by a standard filter after dilution with ambient air at a temperature of 125.degree. F. Included in this definition are, conglomerated carbon particles, absorb hydrocarbons, including known carcinogens and sulfates.
These particulates are very small in size having a mass medium diameter of 4-12 micro inches, and are extremely light weight. During the life of the typical vehicle, approximately 20 cubic feet of particulate matter which must be trapped will be emitted per 100 thousand miles of engine operation.
Presently, ceramic materials are being used to filter these gas entrained carbon particulates from the exhaust streams of diesel engines. These filters and filtering methods rely extensively on the porosity of the filter and the gas permeation through the filter walls. A major concern when utilizing such particulate filters in diesel engine exhaust streams, is to obtain the largest possible trapping area or trapping volume so as to minimize a build-up of back pressure in the exhaust system. At the present time, one of the most effective filters in achieving the above stated goals is a honeycomb extruded structure formed of an extruded ceramic material as is shown in U.S. Pat. No. 4,464,185 issued to Tomita et al. Therein, a honeycomb structure is formed by extruding a slurry material to form a body having a plurality of passageways therethrough. Once this body has been obtained the ends of the honeycomb structure are covered and inlet and outlet passages are formed in an alternate manner, i.e. an alternate matrix of opened end and closed end passageways are formed. However, this large and fragile honeycomb structure must be manufactured to meet the specifications of each individual exhaust system and any flaws or subsequent damage to this structure will render the filter unusable.
One solution to the above mentioned problem is disclosed in U.S. Pat. No. 4,251,239 to Clyde et al., wherein a ceramic sponge is formed from a plurality of structures which are stacked one upon the other and subsequently dipped into a ceramic slurry and squeezed to remove excess material therefrom, such that only the sponge fibers are coated. However, as above this structure must also be formed to the specification of the specific exhaust system in which it is be installed and consequently any flaws or damage to the structure will render the structure as a whole unuseable.
In U.S. Pat. No. 4,652,286 to Kusuda et al. an exhaust gas filter for diesel particulates is disclosed which comprises a plurality of corrugated sheets and plain flat sheets which are bonded together to form the filter structure. The ceramic fiber sheets are produced by a paper forming method wherein a ceramic fiber is chopped into short fibers and dispersed into water. A silica-alumina clay is then mixed with the fibers along with an organic binder and an organic fiber to form a slurry. This slurry is then placed in a forming die and vacuum drained to form the above mentioned sheets. These sheets are then fired to form the final product. The exhaust gas containing the entrained particles then flows through the openings in the upstream end of the corrugated sheet through the porous walls of the sheet and exits through the openings in the downstream end of the sheet. Porous sheets of this type by their inherent nature are extremely fragile and may become damaged during their formation or when in use in the exhaust system of a diesel engine resulting in the necessary replacement of the entire structure since the sheets which form this structure are permanently bonded together.
Therefore as is clear from the above, there is a need for an exhaust gas particulate trap which may be formed in a simple manner, which is durable and which can be repaired without the need to replace the entire structure constituting the filter.